A head-mounted display, abbreviated “HMD”, is a display device that is worn on a user's head that has either (1) a single small display optic located in front of one of the user's eyes (monocular HMD), or (2) two small display optics, with each one being located in front of each of the user's two eyes (bi-ocular HMD), for viewing a wide range of visual display content by a single user. A bi-ocular HMD allows for the possibility that the user may view visual content in 3-dimensions. The HMD devices that can currently be found in today's military, commercial, and consumer markets are primarily either goggles/eyeglasses type devices that are worn the way a pair of goggles or eyeglasses are worn, or they are helmet-mounted devices that are attached to a helmet that is worn on the user's head. Additionally, the HMD devices that can currently be found in today's market primarily rely on three different technologies, and thus fall into three different categories; refractive, diffractive, and laser writer.
The first category of HMD devices currently found on the market is the refractive HMD. Refractive HMD's use the optical physics principle of refraction in order to transmit the projection of visual content from a visual display source to a user's eye. Refractive HMD's work by transmitting a projection of visual content from a display source through a light transmission medium, typically a transparent plastic such as acrylic, to produce a final coherent and often magnified image to the user's eye. The light transmission medium is essentially a lens or series of lenses that bend and magnify the light waves from the visual source as they enter and exit the transmission medium so as to form the magnified cohesive image, similar to the operation of a magnifying glass. This is the dominant methodology employed in most HMD's on the market today.
While the refractive HMD may be the dominant methodology used in the HMD market, it does have several drawbacks. The problem with such refractive HMD's is that, with the transmission medium typically being large blocks of heavy plastic located in the optical path of the HMD, this type of HMD is very heavy, bulky, and cumbersome for a user to wear on either his head or face. This limits the overall comfort for the user wearing such an HMD. In addition, such a bulkier fit for the user significantly limits the styling that may be applied to such a device. Furthermore, because the refractive lenses of refractive HMD's are often located in the user's direct field of view, creating a refractive HMD that gives a user adequate “see-through vision,” or the ability to simultaneously see the projected visual content and at the same time clearly see through the projected content to the real-world outside surrounding environment, a “mixed-reality” view, becomes very complicated. Another drawback of refractive HMD's is that they can often prevent a user from seeing anything other than the projected visual content or can severely limit a user's peripheral vision, which can ultimately leave the user feeling claustrophobic. A further drawback of refractive HMD's is that, for those commonly found in the consumer or commercial markets, they have a very limited field-of-view (“FOV”) angle, with the typical FOV being about 25-degrees and the high-end FOV being about 40-degrees. When trying to increase the FOV of refractive HMD's commonly found in the consumer and commercial markets above the typical FOV of 25-degrees, the cost and weight of the device increases dramatically, which can be a significant prohibitive factor in two already competitive markets. This situation is apparent in the military market where refractive HMD's with FOV's between 40-degrees and 120-degrees are much more common, however as previously stated, they are extremely heavy and very expensive.
The second category of HMD devices currently found on the market is the diffractive HMD, or more accurately, a hybrid refractive/diffractive HMD. Diffractive HMD's use the optical physics principle of diffraction and diffraction gratings as well as refraction in order to transmit the projection of visual content from a visual display source to a user's eye. With this type of HMD, the projection of the visual content is passed through both a transmission medium and a diffraction grating contained within one of the refractive transmission medium elements to produce a final coherent and often magnified image to the user's eye. The light waves from the projected visual content that are passing through the transmission medium ultimately pass through the diffraction grating, which serves to present a single coherent image to the user. The main drawback to such hybrid HMD systems is that they require a high intensity light source and therefore they are very inefficient when it comes to power consumption, they consequently require a substantial amount of power to operate at acceptable levels, and they have a significantly reduced display lifetime. Additionally, they have somewhat limited FOV capabilities due to the physics of how diffraction gratings operate.
The third category of HMD devices currently found on the market is the laser-writer HMD. The laser-writer HMD uses a remote laser light engine, often consisting of a triad of red, green, and blue lasers, and a set of laser writers to bend and beam the laser lights, according to an input visual display signal, into a coherent visual image. The lasers and laser writer are connected to a head mounted display unit by coherent fiber optic cable in order to transmit the images to the head mounted unit. The images are then projected from the coherent fiber optic cable onto the final viewing screen, typically a transparent lens in the HMD unit, for viewing by the user. One drawback associated with this type of HMD is that the coherent fiber optic cable required for such a system is very expensive. Another downside to such HMD systems is that, as the image comes out of the fiber optic cable, the head unit will still need some type of refractive optic to magnify the image, which in turn translates to a limited FOV and increased weight of the head unit. Furthermore, another downside related to laser-writer HMD's becomes apparent when using such a system to view visual content in 3D. To do so, the HMD system would either be required to beam two distinct images to the head unit at the same time over a single fiber optic cable, thus requiring the head unit to incorporate a beam splitter to separate the two images for each eye, or the HMD system would require a second laser system working simultaneously with the first laser system in order to produce the second image necessary to deliver 3D visual content. In either case, this can become extremely expensive. An additional downside to the laser-writer HMD device is that the power consumption necessary to run such a device is extremely high. Lastly, transmitting an image to the head mounted unit via fiber optic cables can be potentially problematic if care is not taken to observe the required minimum bend radius of the fiber optic cable. If the cable is bent at too tight a radius, this will result in significant signal losses.
None of the three categories of HMD systems that are available today are capable of providing magnified coherent visual content for viewing by a user from a single device that is all at once inexpensive, lightweight, comfortable, and that can be considered a near-to-eye HMD device. Consequently, because of the shortcomings and problems associated with the three types of systems currently available, there is a need in the industry for a new type of HMD device that is fairly inexpensive, lightweight, compact, comfortable, and is a near-to-eye device.